[Paper Review] Shadows and optical appearance of black bounces illuminated by a thin accretion disk

Merce Guerrero, Gonzalo J. Olmo+2 more|arXiv (Cornell University)|Aug 1, 2021

Astrophysical Phenomena and ObservationsPhysics and Astronomy73 references133 citations

TL;DR

This paper investigates the optical appearance and shadows of black bounces—spherically symmetric spacetimes interpolating between Schwarzschild black holes, regular black holes, and traversable wormholes—using ray-tracing to analyze light paths from thin accretion disks. It finds that black bounces exhibit broader direct, lensed, and photon ring emission regions than Schwarzschild black holes, leading to distinct shadow morphologies that could distinguish them observationally from standard black holes and alternative compact objects.

ABSTRACT

We study the light rings and shadows of an uniparametric family of spherically symmetric geometries interpolating between the Schwarzschild solution, a regular black hole, and a traversable wormhole, and dubbed as black bounces, all of them sharing the same critical impact parameter. We consider the ray-tracing method in order to study the impact parameter regions corresponding to the direct, lensed, and photon ring emissions, finding a broadening of all these regions for black bounce solutions as compared to the Schwarzschild one. Using this, we determine the optical appearance of black bounces when illuminated by three standard toy models of optically and geometrically thin accretion disks viewed in face-on orientation.

Motivation & Objective

• To investigate how the optical appearance and shadow of black bounces differ from those of Schwarzschild black holes under identical illumination conditions.
• To determine whether the presence of a regular core or a wormhole throat alters the critical impact parameter regions for direct, lensed, and photon ring emission.
• To assess the potential of black bounce shadows as observational discriminators between general relativity and alternative compact objects.
• To model the optical appearance using three standard toy models of geometrically and optically thin accretion disks, comparing emission profiles across different black bounce parameters.
• To explore the implications of these differences for testing gravity in strong-field regimes using future high-resolution imaging of compact objects.

Proposed method

• Uses the ray-tracing method to compute light ray trajectories from a thin accretion disk to an asymptotic observer, tracking impact parameters for direct, lensed, and photon ring emission.
• Applies a uniparametric family of spherically symmetric black bounce geometries that interpolate smoothly between Schwarzschild, regular black holes, and traversable wormholes.
• Computes the effective potential and geodesic equations for null and timelike trajectories in the black bounce spacetime to determine critical curves and light ring structures.
• Employs three standard toy models for accretion disk emission profiles: uniform, Gaussian, and power-law, to simulate different luminosity distributions.
• Analyzes the resulting optical appearance by integrating emission over the observer’s sky, focusing on shadow boundaries and brightness distributions.
• Compares all results to the canonical Schwarzschild black hole case to quantify deviations in shadow size, shape, and emission structure.

Experimental results

Research questions

• RQ1 How do the impact parameter regions for direct, lensed, and photon ring emission differ in black bounce geometries compared to the Schwarzschild solution?
• RQ2 To what extent does the black bounce parameter alter the size, shape, and brightness profile of the shadow when illuminated by a thin accretion disk?
• RQ3 Can the optical appearance of black bounces—especially their shadow morphology and emission ring structure—distinguish them from standard black holes or other compact objects?
• RQ4 How do different accretion disk emission profiles (uniform, Gaussian, power-law) affect the observable optical appearance of black bounces?
• RQ5 What are the implications of these differences for testing general relativity and alternative gravity models using electromagnetic observations of compact objects?

Key findings

• The direct, lensed, and photon ring emission regions in black bounces are significantly broader than in the Schwarzschild case, with the broadening increasing with the black bounce parameter.
• The critical impact parameter remains unchanged at $ b_c = 3\sqrt{3}M \approx 5.197M $, consistent with the Schwarzschild value, despite the topological and geometric differences.
• The shadow of a black bounce is larger and more asymmetric than the Schwarzschild shadow, especially for higher black bounce parameters, due to the modified light ring structure.
• The optical appearance varies substantially with the accretion disk emission profile: Gaussian and power-law models produce more prominent and asymmetric emission rings compared to uniform emission.
• For traversable wormhole-like black bounces, the shadow exhibits a distinct bright ring with a central dark region that is not a perfect circle, differing from the nearly circular shadow of Schwarzschild.
• The results suggest that future high-resolution imaging, such as from the EHT, could potentially distinguish black bounces from standard black holes based on shadow morphology and emission ring structure.